Nitric acid is a strong inorganic acid that has many industrial uses. Its principal use is for the production of fertilizers and explosives. Large quantities of nitric acid are produced during thunderstorms and fall to the earth in rain. The rain falls as a very weak solution of nitric acid. The production of nitric acid during thunderstorms allows nitrogen from the air to become part of the soil in a form that plants can use (see Nitrogen cycle ). Nitric acid was one of the first acids known. Many alchemists of the Middle Ages used it in their experiments.
Nitric acid is such a powerful oxidizing agent that it dissolves many metals. But it does not attack gold and platinum. A drop of nitric acid on a piece of jewelry tells whether it is made of genuine gold or platinum. These two metals can be dissolved by aqua regia, a mixture of nitric acid and hydrochloric acid (see Aqua regia ).
Nitric acid is used to manufacture ammonium nitrate, an ingredient of many fertilizers, and to make explosives, flares, and rocket propellants. The chemical industry uses nitric acid to prepare nylon and many organic compounds that are used as dyes and drugs. Nitric acid reacts with toluene in the presence of sulfuric acid to form trinitrotoluene, better known as TNT.
Commercially, most nitric acid is produced by oxidizing ammonia using a platinum catalyst. Ammonia and air are passed through heated platinum gauze. The gases react to form water, nitric oxide, and nitrogen dioxide. Upon cooling, this gaseous mixture forms nitric acid. This method is called the Ostwald process, after Wilhelm Ostwald, the German chemist who developed it. Nitric acid is also produced by heating saltpeter with sulfuric acid. In this process, nitric acid is recovered by distillation.
Nitric acid is a colorless liquid with a suffocating odor. It develops a yellow color if kept in bottles that are not tightly stoppered. This is due to nitrogen dioxide gas that results from decomposition of the acid. Nitric acid stains the skin yellow and can cause burns. The metal salts of nitric acid, called nitrates, are soluble in water.
